In the oral hygiene field the so-called interdental cleaning brushes are widespread, which are generally constituted by a suitably shaped supporting shank, referred to as “handle”, having a free end on which a small elongated brush body is mounted, briefly referred to as “brush”.
Essentially, two kinds of brush-like devices are commercially available and, namely, a “standard” type, whose handle has practically the sizes of a toothbrush and has an interchangeable brush, and a “pocket” type, whose handle is considerably smaller and conceived as single disposable element.
The active part, i.e., the brush itself, comprises a thin stem portion from which a plurality of bristles radially branches for cleaning action. The stem is typically in the form of a twisted pair of metal wires, possibly coated with plastic material, among which turns, the bristles remain locked. An appendix part of the stem is free from bristles and constitutes the so-called “tang”, which is used for the engagement with the handle. The bristles project towards a reference surface which may take many forms, but typically defines an elongated rotational body.
For obvious hygienic and functional reasons, it is appropriate that the brush is frequently replaced. To this end, at least in standard devices, it is normally provided an engagement head arranged so as to allow an easy but safe assembly and disassembly of the brush (in order to avoid that the brush accidental detaches and injures the user or is ingested in the mouth). In particular, the provisional fastening of the brush is also achieved by exploiting the large deformability of the tang.
Conversely, in the pocket devices, for sake of constructional economy and low economic impact of the handle, the brush is embedded in the handle head and it is not replaceable. This implies that the fastening system of the brush must be simpler and more economical from the manufacturing point of view.
Traditionally, as can be imagined, the tang of the brush is hence embedded in the handle head part during the manufacturing phase, by simply inserting the tang into the mould, and moulding on it the plastic material constituting the handle.
However, this construction method entails some technical problems.
First, there is a general adaptation problem of the mould to the size of the brush stem. In fact, since the brush derives from a twisting process of two metal wires, it is not possible to ensure close tolerances; furthermore, it would be desirable to use different brushes having the same handle, but this involves providing a plurality of moulds only to accommodate the different diameters of the tangs.
Furthermore, the brush stem is retained in a through seat of the mould which, having theoretically a circular profile, cannot perfectly match the real tang section which, instead, derives from the profile of twisted metal wires. Therefore, considering the natural coupling tolerances, gaps between stem and shape of the mould are formed which allow a partial leakage of the molten plastic material, which is injected at high pressure into the mould, with an unwanted formation of moulding burrs.
To partially solve this problem, it has been proposed to shape the mould with an expansion chamber at the insertion position of the tang, in order to locally reduce the pressure and eliminate the burr problem. However, this leads to the formation of a small spherical body at the handle end, which does not meet the users' consent.
Alternatively, it is possible to size the seat of the mould so that it strongly clamps the stem of the brush: this solution, however, leads to a localized hardening or yield of the metal wires, which, then, lose resiliency. Therefore, when the brush is subjected to a lateral force, tends to remain folded on one side, which represents a defect.
This problem has already been tackled in the prior art, by providing the insertion of specific gaskets on the tang, in the position where the mould should close. For example, U.S. Pat. No. 8,607,398 discloses a construction method in which a plastic deformable ring is mounted on the tang, which has also a tapered cross-section to better lock, in the closing area of the mould.
However, this technique involves a series of problems, both to handle and mount the sealing ring on the tang, and to ensure that its position is always centered in the desired position.
But above all, there is the additional problem of being able to maintain the extension of naked tang (i.e. the portion between the bristles and the head end of the handle) as short as possible, both for aesthetic reasons and for functional reasons (to reduce the length of the stem which remains without a lateral support). With a traditional mould, this extension cannot be reduced beyond a certain extent, because the mould wall occupies a certain space. Therefore, different drawbacks are resulting, including the fact that the stem tends to remain deformed if excessively folded.
A construction technique, alternative to that of the moulding on the stem, provides the hot insertion of the stem in the plastic handle head. Also this construction method, which is not contemplated within the scope of the present application, shows other drawbacks always related to the effectiveness of the brush and the resulting aesthetics of the whole device. Moreover, also with this technique, the stem is not adequately supported and it is necessary to provide elastic support systems that complicate the construction structure, as illustrated, e.g., in EP 1862090.
All these problems are also found in the field of cosmetic applicators, e.g., mascara brushes, or for medical use, wherein the engagement of the stem bristled on the handle or retention portion, of plastic material, shows strong analogies with the sector of the interdental brushes.
In the following, we will thus refer to a brush-like device, being understood that the teaching of the invention may apply to any one of the application sectors mentioned above.